Aldehydes are among the most widely occurring compounds in nature and the chemical industry. Aldehydes are produced through a variety of chemical reactions, including, for example, oxidation of primary alcohols, ozonolysis of alkenes having at least one vinylic hydrogen, or partial reduction of certain esters (e.g., with DIBAH, diisobutylaluminum hydride). As a reactant, an aldehyde can be oxidized to yield a carboxylic acid or an ester through reversible nucleophilic addition of water or an alcohol. Nucleophilic addition of water to the carbonyl group initially yields a gem diol intermediate, while a hemiacetal intermediate is formed on the nucleophilic addition of an alcohol.
Aldehydes have become undesired byproducts of the chemical industry because of environmental regulations. In particular, regarding the present disclosure, methods of acetic acid manufacture produce undesired aldehydes in process streams. For example, methods of acetic acid manufacture, including those accomplished under high-water (e.g., see U.S. Pat. No. 3,769,329) or low-water (e.g., see U.S. Pat. No. 5,001,259) conditions, utilize the carbonylation reaction of methanol (MeOH) with carbon monoxide (CO). MeOH and CO are contacted with a catalyst solution: acetic acid, rhodium (Rh) complex, a promoter (methyl iodide, Mel), and a Rh catalyst stabilizer--lithium iodide (Lil). Byproducts of the overall reaction include hexyl iodide, carbon dioxide (CO.sub.2), propionic acid, and aldehydes. An iridium catalyzed methanol carbonylation process is also described in EP 0752,406. Byproducts of the overall reaction differ slightly from the rhodium catalyzed carbonylation process. However, aldehyde impurities remain a concern. Aldehyde impurities of major concern include acetaldehyde (AcH), crotonaldehyde, and 2-ethylcrotonaldehyde. The latter two aldehyde impurities are each derived from AcH, and each appear to increase in amount as the amount of water decreases in the carbonylation reaction mixture (U.S. Pat. No. 5,723,660).
The present invention is not directed to any particular chemical process, or to the production of AcH by a particular route, but to the handling of the AcH byproduct. Accordingly, treatment or processing of AcH is important to this invention. Discussion of carbonylation processes is included to help exemplify the electrooxidation of the AcH invention.
Lowering concentrations of AcH in process streams would facilitate compliance with applicable environmental regulations. Furthermore, since other impurities--including propionic acid, crotonaldehyde, and 2-ethylcrotonaldehyde-are derived from AcH, lowering concentrations of AcH in process streams would also further the goal of lowering concentrations of these derivative impurities. While distillation may be used to remove AcH from process streams, exclusive use of distillation has disadvantages. The expense of building a distillation unit or tower that meets environmental regulations for containment of AcH is among the disadvantages.
A chemical process of addition of AcH with hydroxylamine sulfate has been tried as a method for removing AcH from process streams. However, the process led to many side reactions that created additional problems.
To develop processes for removing aldehydes, including AcH, from process streams, electrochemical oxidation of aldehydes to carboxylic acids or esters may be considered. For example, U.S. No. Pat. 4,450,055 (incorporated herein by reference) describes an electrochemical process involving partial oxidation of ethane to AcH, some of which is further oxidized to acetic acid.
Electro oxidation reactions have been reported for the electrosynthesis of methyl ester (RCOOMe) from aldehyde (RCHO) in divided cells. One electro oxidation reaction is carried out in the presence of MeOH and sodium cyanide (NaCN): ##STR1##
Precent yield of esters, based on aldehyde oxidized, has been reported to be between 48% to 83% for various substituted benzylaldehydes, 38% for octyl aldehyde, and roughly 44% for butyraldehyde (Chiba et al., 1982, Bull. Chem. Soc. Jpn. 55: 335-36). Another electro oxidation reaction is carried out in the presence of sodium methoxide (NaOMe), MeOH, and potassium iodide (KI): ##STR2##
Percent yield of esters, based on aldehyde oxidized, has been reported to be between 68% for 2,2-dimethyl-3-hydroxypropionaldehyde to 91% for 4-nitrobenzaldehyde (Okimoto and Chiba, 1988, J. Org. Chem 53: 218-19).
Shono et al. have noted electrochemical oxidation of aldehydes to esters in the presence of MeOH or butanol containing KI or KBr in undivided cells (Shono et al., 1985. J. Org. Chem. 50: 4967-4969). Using MeOH as the reactant alcohol, the present inventors repeated the unbuffered process of Shono et al. as described herein in Example 1 and found it to be unworkable for lower chain alkyl aldehydes. When AcH was used as the aldehyde substrate, the inventors recovered primarily ethanol (EtOH), acetic acid, and MeI from the AcH-MeOH-KI reaction mixture, not the ester methyl acetate (MeOAc). However, when aldehydes containing five or more carbon atoms (such as valeraldehyde, hexanal, and decyl aldehyde) were used as substrates, the expected methyl esters were recovered using the unbuffered process of Shono et al.
Published Japanese Application No.: Sho 55-54583 (Shono) (disclosed Apr. 21, 1980) also has noted the electro oxidation of aldehydes in the presence of alcohol and iodide as a method for manufacture of various esters. This publication reportedly provides for dripping a solution of hexanal, n-butanol, and n-hexane into an aqueous solution of concentrated KI (1 mole KI per liter). This process is not adaptable to an industrial scale.
The present inventors provide herein an improved process for electro oxidation of an aldehyde to an ester. The process is readily adapted to an industrial scale.
Abbreviations AcH Acetaldehyde amu atomic mass unit CHI.sub.3 Iodoform CO Carbon Monoxide CO.sub.2 Carbon Dioxide 1,1'-DME 1,1'-Dimethoxyethane EtI Ethyl Iodide EtOH Ethanol GC Gas Chromatography HCHO Formaldehyde HI Hydriodic Acid HPLC High Pressure Liquid Chromatography KI Potassium Iodide Li Lithium LiI Lithium Iodide LiIO.sub.3 Lithium Iodate LiOAc Lithium Acetate mA milliamperes MeI Methyl Iodide MeOAc Methyl Acetate MeOH Methanol MS Mass Spectroscopy % Covers. Percent Conversion % R.E. Percent Relative Efficiency NaCN Sodium Cyanide NaOMe Sodium Methoxide RCHO Aldehyde RCOOMe Methyl Ester Rh Rhodium Wt. % Weight percent